Reverse osmosis or nanofiltration membranes are used to separate dissolved or dispersed materials, i.e., solute, from a solvent or a dispersing medium, e.g., water. This is accomplished because the membranes are selectively permeable to certain components of the mixture to be separated. Usually, water is the component to which such membranes are permeable. The separation process typically involves bringing an aqueous feed solution into contact with one surface of the membrane under pressure so as to effect permeation of the aqueous phase through the membrane while permeation of the dissolved or dispersed materials is prevented.
Both reverse osmosis and nanofiltration membranes usually have a discriminating layer fixed to a porous support and are referred to as composite membranes. Ultrafiltration and microfiltration membranes may also have a composite arrangement. The support provides physical strength but offers little resistance to the flow rate due to its porosity. On the other hand, the discriminating layer is less porous and provides for the rejection of the dissolved or dispersed materials. Therefore, it is generally the discriminating layer which determines the rejection rate, i.e., the percentage of the particular dissolved material that is rejected, and the flux, i.e., the flow rate at which solutions pass through the membrane.
Reverse osmosis membranes and nanofiltration membranes vary from each other with respect to their degree of impermeability to different ions and organic compounds. Reverse osmosis membranes are relatively impermeable to virtually all ions, including sodium chloride. Therefore, reverse osmosis membranes are widely used for the desalination of brackish water or seawater to provide relatively non-salty water for industrial, commercial, or domestic use because the rejection rate of NaCl for reverse osmosis membranes is usually from about 95 to about 100 percent.
On the other hand, nanofiltration membranes are usually more specific for the rejection of ions. Generally, nanofiltration membranes reject divalent ions, including radium, magnesium, calcium, sulfate, and nitrate. In addition, nanofiltration membranes are generally impermeable to organic compounds having molecular weights above about 200. Additionally, nanofiltration membranes generally have higher fluxes than reverse osmosis membranes. These characteristics render nanofiltration membranes useful in such diverse applications as the "softening" of water and the removal of pesticides from water. As an example, nanofiltration membranes generally have a NaCl rejection rate of from about 0 to about 95 percent but have a relatively high rejection rate for salts such as magnesium sulfate and in some cases organic compounds such as atrazine.
Among particularly useful membranes for reverse osmosis and nanofiltration applications are those in which the discriminating layer is a polyamide. The polyamide discriminating layer for reverse osmosis membranes is often obtained by an interfacial polycondensation reaction between a polyfunctional aromatic amine and a polyfunctional acyl halide as described in, for example, U.S. Pat. No. 4,277,344, which is incorporated herein by reference. In contrast to reverse osmosis membranes, the polyamide discriminating layer for nanofiltration membranes is typically obtained via an interfacial polymerization between a piperazine or an amine substituted piperidine or cyclohexane and a polyfunctional acyl halide as described in U.S. Pat. Nos. 4,769,148 and 4,859,384. Another way of obtaining polyamide discriminating layers suitable for nanofiltration is via the methods described in, for example, U.S. Pat. Nos. 4,765,897; 4,812,270; and 4,824,574. These patents describe changing a reverse osmosis membrane, such as those of U.S. Pat. No. 4,277,344, into a nanofiltration membrane.
Membrane fouling is a concern in many membrane applications. "Fouling" describes the collection of debris and organic material on the membrane surface which results in reduced flux. This material typically includes tannic and humic acids, proteins, carbohydrates, bacteria and other organic material commonly found in water sources. Surfactants can also be a significant source of membrane fouling. These materials associate with the membrane surface and reduce membrane solubility, resulting in reduced flux.
S. Belfer et al. in the Journal of Membrane Science, volume 139, no. 2, pages 175-181 (1998) describes a method for inhibiting membrane fouling by radically grafting methacrylic acid or polyethylene glycol methacrylate directly on the polyamide surface of commercial RO composite membranes, (e.g., "FT-30 BW.TM.", available from FilmTec Corporation). This method requires the use of redox initiators (potassium persulfate-sodium metabisulfite) to form a radical species which subsequently react with the membrane surface to form a graft therewith. Unfortunately, once the radical is formed, it is very reactive and must be contacted with the membrane's surface shortly after being formed; otherwise the radical will react with others species present rather than the membrane surface. As a consequence, this process is difficult to adapt to commercial scale and requires additional handling of chemical initiators.
Thus, methods are sought for modifying the surface of composite membranes to reduce fouling. Moreover, methods are sought which are adaptable to commercial scale membrane fabrication, preferably without the use of chemical initiators. Furthermore, it is desired that such membranes achieve reduced fouling without significantly compromising membrane performance in terms of flu and solute passage (e.g., salt passage).